High-field magnetization, field-dependent specific heat measurements, and zero-field inelastic magnetic neutron scattering have been used to explore the magnetic properties of copper pyrazine dinitrate $\left[\mathrm{Cu}\right({\mathrm{C}}_4{\mathrm{H}}_4{\mathrm{N}}_2\left)\right({\mathrm{NO}}_3{)}_2].$ The material is an ideal one-dimensional spin-1/2 Heisenberg antiferromagnet with nearest-neighbor exchange constant $J=0.90\mathrm{}\left(1\right)\mathrm{}\hspace{0.5em}\mathrm{meV}$ and chains extending along the orthorhombic a direction. As opposed to previously studied molecular-based spin-1/2 magnetic systems, copper pyrazine dinitrate remains gapless and paramagnetic for $g{\mu }_{B}H/J$ at least up to 1.4 and for $k_{B}T/J$ at least down to 0.03. This makes the material an excellent model system for exploring the $T=0\mathrm{}$ critical line that is expected in the $H-T$ phase diagram of the one-dimensional spin-1/2 Heisenberg antiferromagnet. We present accurate measurements of the Sommerfeld constant of the spinon gas versus $g{\mu }_{B}H/J<\mathrm{}1.4\mathrm{}$ that reveal a decrease of the average spinon velocity by 32% in that field range. The results are in excellent agreement with numerical calculations based on the Bethe ansatz with no adjustable parameters.

• Received 27 August 1998

DOI:https://doi.org/10.1103/PhysRevB.59.1008